This invention refers to a heat exchanger and, more particularly, to a heat exchanger for the processing of food or pharmaceutical products in utmost sanitary conditions.
In many industrial processes, and specially in the food industry, it is necessary to heat or cool large volumes of a fluid by absorbing heat from or transferring heat to another fluid which is at a higher or lower temperature, respectively.
The most common heat exchangers comprise a cluster of straight, helical or serpentine tubes arranged inside an enclosure or shell. A first fluid flows through the tubes while a second fluid flows back and forth across the tubes between baffles. Heat exchange between the first and second fluids takes place across the walls of the tubes.
The quantity of heat transferred is governed by three main factors; (a) the extension and nature of the heat transfer surface exposed to both fluids; (b) the overall coefficient of heat transfer from one fluid through the intervening wall to the other fluid; and (c) the mean temperature difference across the intervening wall from one fluid to the other.
The first item depends upon the number of tubes employed and their length. The second depends upon the resistance to the flow of heat created by the tube walls and the thin films of stagnant fluid on either sides of the walls. The third factor depends upon the difference in temperature between the first and second fluids at the inlet and exit to the exchanger.
The overall coefficient of heat transfer depends, to a large extent, upon the film coefficients of the stagnant fluid layers. The important physical properties which affect film coefficients are thermal conductivity, viscosity, density and specific heat. Factors within the control of the designer include velocity of flow, and shape and arrangement of the heating surface.
For the first fluid flowing through the tubes, the velocity is determined quite precisely by the flow rate and the number and diameter of the tubes. The velocity of the second fluid, which flows inside the shell across the tubes, also depends on the flow rate and the passage sections defined among the tubes, but flow conditions may vary considerably from one area to another of the exchanger.
Since for a given heat exchange area, the exchanger efficiency is substantially improved when the velocity of the second fluid increases, several designs have been proposed wherein the second fluid also circulates through channels of controlled cross section at high velocity and in turbulent flow conditions in intimate contact with the tube or tubes through which the first fluid circulates. However, such designs are complex and of costly construction, or difficult to disassemble and re-assemble and/or have unaccessible or rugous surfaces which cannot be cleaned with simple methods or inspected visually in order to ensure that they strictly adhere to adequate sanitary conditions. Therefore, these known heat exchangers are not intended nor adapted for use in applications where thorough and frequent cleaning of the internal parts of the exchanger is required nor in processes which do not tolerate even minute amounts of contaminants.
Thus, convencional heat exchangers must be cleaned with chemicals of energic action, for instance by circulating a hot nitric acid solution through the exchanger. This procedure is not desirable inasmuch as the use of chemicals does not ensure complete elimination of solid particles which may be retained or entrapped inside the exchanger. Furthermore, some of these chamicals may attack the metal surfaces of the exchanger, or the sealing gaskets, or leave contaminant residues.
On the other hand, the food industry is essentially seasonal, and the necessity often arises of treating food products of different nature and which should be processed at different operative conditions. Since known heat exchangers are designed for specific process requirements, a change in the product to be treated imposes the need of using a different exchanger with the attending capital investment.
Therefore, it is desirable to provide an efficient heat exchanger of simple construction, easy to clean and which could be adapted, at a minimum cost, to the treatment of fluids having different viscosities and specific gravity and requiring different flow rates, velocities, residence times and relative flow directions.
Among the heat exchangers of the prior art, the following are mentioned:
French Pat. No. 2155770 discloses a heat exchanger wherein a first fluid flows through a helically wound tube arranged between two walls of revolution in order to define, between the tube coils, another helical path for a second fluid. The heat transfer takes place across the wall of the helical tube.
In the exchanger of the above French patent, one of the fluids must flow through a helical tube the interior of which is obviously unaccessible. Besides, the exchanger of this patent cannot be disassembled easily and cleaning of the outer surface of the helical tubes would be too difficult or time-consuming. Furthermore, the heat exchanger of this patent is of complex and costly construction.
German Pat. No. 1111654 employs a similar concept. A helically corrugated tubular element is arranged between an inner and an outer cylindrical walls so as to define a first helical path for a first fluid between the outer wall and the corrugated element, and a second helical path for a second helical path for a second fluid between the corrugated element and the inner wall. The heat transfer takes surfaces. Finally, these exchangers have complex inlet and outlet channels which are difficult to disassemble and have unreliable seals at which the interacting fluids may contact accidentally.
None of the above patents disclose a heat exchanger in which the flow conditions of the interacting fluids may be changed to adapt them to specific requirements.